A collaborative program utilizing the disciplines of biochemistry, genetics and immunology has been designed to explore the molecular causes of the immunodeficiency diseases of childhood. Recent studies have shown that a deficiency of adenosine deaminase in the purine pathway is responsible for one form of severe combined immunodeficiency disease. This proposal will amplify these findings and extend the investigations to other enzymes which, when deficient, may interfere with the normal cellular maturation required for immunological competence. Selected enzymes will be studied in short term lymphocyte cultures, cultured skin fibroblasts and specific organ tissues. The methodology will be extended to screening techniques for the detection of enzyme deficiencies, family studies for the detection of heterozygotes, cell culture systems for the in vitro expression of the disorder, and to cultured amniotic fluid cells for use in prenatal diagnostic programs. Adenosine deaminase will serve as the primary focal point of the study. Other enzymes of purine and pyrimidine biosynthesis will be systematically evaluated in children with immune deficiency states (UMP kinase, guanine deaminase, adenylate deaminase and adenosine kinase are examples). The mutant enzymes will be characterized by their kinetic and electrophoretic properties, heat stability and reaction to inhibitors. Antibodies will be prepared using purified normal enzymes for testing against mutant enzymes. Correlations between the enzyme alteration and the clinical phenotype of the immune state will be done to establish biochemical or clinical heterogeneity. When metabolites are found to accumulate as a consequence of an enzymatic deficiency they will be tested in fibroblast and lymphoid culture systems to establish their potential role as inhibitors of the normal physiological responses of immunity. All information will be integrated towards the goal of understanding the basic defects in the immune disorders of childhood, their early detection, specific therapy, and prevention through genetic counseling or prenatal detection.